1. Field of the Invention
The present invention relates to a compact variable focal length optical system which is suitable for a high zoom ratio.
The invention also relates to a variable focal length optical system which is capable of shifting images. Such images are shifted by moving a sub-lens group in a direction almost orthogonal to the optical axis.
2. Description of Related Art
Zoom lenses are becoming popular in phototaking optical systems for still cameras, video cameras, etc. A major trend is moving toward cameras which have lenses providing a zoom ratio of over 3 times. These lenses are referred to as zoom lenses with high zoom ratios.
Such a camera normally uses a 35-mm zoom lens with which a field angle having a focal length of about 50 mm is included in its focal length range. A so-called multiple group zoom lens which is constructed by three or more movable lens groups for zooming is used for high zoom ratio zoom lenses.
Also, integrated cameras, in which a phototaking optical system and a camera main body are integrally constructed, should be portable, small in size and light in weight. For this reason, various proposals have been made for zoom lenses to contribute to small size and light weight. In such zoom lenses, a long focal length accompanies a zoom ratio increase.
If the shutter speed is slow, moreover, then a blurred image is produced during exposure due to vibrations of the camera caused by hand-held vibrations, etc. Poor photography results.
In general, it is known that an image is shifted when a lens group forming a part of the lens groups constituting a lens system (hereinafter denoted to as "a shift lens group") is moved in a direction approximately orthogonal to the optical axis. An optical system with which an excellent imaging performance can be obtained, even when a shift lens group is shifted, is referred to as an optical system which is capable of shifting images.
A so-called vibration reduction optical system, which is constructed with an optical system capable of shifting images, a vibration sensor for sensing vibrations of the optical system and outputting data on the vibrations, and a driving system for moving a shift lens group, has been used in order to solve the problems of poor photographing due to the hand-held vibrations mentioned above. The vibration reduction optical system shifts an image by moving a shift lens group through a driving system so that vibrations of the optical system caused by hand-held vibrations and so on are sensed by a vibration sensor and deviations of an image point, caused by the vibration of the optical system, are canceled. Thus, in the vibration reduction optical system, deviations of an image point caused by the vibration of the optical system, that is, a blurred image, can be corrected with a forcefully produced image shift.
In conventional high zoom ratio zoom lenses, however, if the focal length becomes long in the maximum telephoto state for high zoom ratio, the entire length of the lens system increases and the diameter of a stop increases. This results in a large-size lens system and a large-size camera with lost portability.
Moreover, because it is difficult to hold compact and light cameras without vibrations, it is easy to cause vibrations when pressing a release button; consequently, blurred images are produced in many cases. In particular, in photographic lenses having a long focal length provided for increasing a variable magnification ratio, even a small vibration of a lens system easily generates a largely blurred image, resulting in poor photographing.
In this situation, it is possible, by installing a vibration reduction optical system in a camera, to correct deviations of an image point due to vibrations of a camera caused by hand-held vibrations. However, vibration reduction optical systems have excessive restrictions because of aberration corrections. This requires a long overall lens system and a large aperture stop diameter, making the entire lens system large. Consequently, the portability of a camera body is further reduced.